Hi, Al, I'm not sure I understand you right, perhaps I've missed part of the argument. What evidence has been presented (and by whom) to show that pyrethroid-resistant varroa are not different genetically from susceptible strains? In Europe, pyrethroid-susceptible and pyrethroid-resistant varroa mites exist side by side, within the same population. The resistant individuals arose from the "original" susceptible population due to high selection pressure through overuse of pyrethroids. However, not all varroa in Europe are resistant, there is a cost involved and where the selection pressure is low, pyrethroid-resistance will not appear. The resistant individuals have more or less the same gene pool as the susceptibles but, like all biological material there is some natural variation. Not all susceptible mites have the ability to become resistant as they do not posess the correct character make-up. Those individuals posessing a character for pyrethroid-tolerance at higher doses are selected for when there is increased selection pressure (presence of pyrethroid). There is bound to be genetic variability at some level in varroa mites as with other species. Even with inbreeding there are occasionally mutations. We looked at the potential resistance mechanisms involved and a paper was published by three of my team: Ref: Elke Hillesheim, Wolfgang Ritter and Denis Bassand (1996). First data on resistance mechanisms of Varroa jacobsoni (Oud) against tau-fluvalinate. Experimental & Applied Acarology, 20 (1996) 283-296. At least in Western Europe, the resistance is in part due to an increased ability in the mite to detoxify the pyrethroid toxin. More detail is given in the paper. If anyone on the list is aware of any further work that has been done on the resistance mechanism(s) please contact me. Max Dr Max Watkins Vita (Europe) Limited Tel 44 (0) 1256 473177 Brook House, Alençon Link, Fax 44 (0) 1256 473179 Basingstoke, Hampshire RG21 7RD, UK